Are there any adapters available for using the mini on 12 volt DC?
I can buy stabilised power DC-DC converters (input 10..24 volt) output stable 12,5 volt.
But what is the best way to reduce the 12 volt to 5 volt at 2,5 or 3A what is needed for the AC-Ryan?
The only equipment I can find is usb-adapters for cars for charging phones or mp3 players but max current is then 1A or 1,5A.

Can anybody advise me?
regards
Bob.

Amazon will be a big help or your local hardware. If you're going to modify the power supply, you can try to contact a professional Electronic Technician to help you, specially if you're going to install it in a boat or in a car.